The Periodic Law History of the Periodic Table

Mendeleev’s Periodic Table Classification of elements depended upon accurate measurements of atomic mass. Mendeleev’s first table was published in Elements were arranged in order of increasing atomic mass and similar chemical properties occurred at regular intervals. Approximately 60 elements were known at the time. Mendeleev left empty spaces in his table and predicted the properties of elements that had yet to be discovered.

Mendeleev’s Periodic Table Unanswered Questions: 1.Why could most elements be arranged by increasing atomic mass but others could not? 2.What was the reason for chemical periodicity?

Moseley and the Periodic Law (1911) – Mosely discovered that elements fit into patterns when they were arranged by increasing nuclear charge (# of protons) The concept of atomic number became recognized as a means for identifying and classifying elements. This organization matched Mendeleev’s grouping by chemical properties Periodic Law – physical and chemical properties of the elements are periodic functions of their atomic numbers.

The Modern Periodic Table Noble Gases Discovered between Placed on the periodic table between Group 17 and Group 1 Unreactive elements He Ne Ar Kr Xe Rn

The Inner Transition Metals Lanthanides Discovered in the early 1900’s Similar chemical and physical properties Form 3+ ions Actinides Radioactive elements Some are naturally occurring; others have been created synthetically